The invention relates to an electromechanical transducer having a substantially elliptical vibration. The above transducer is utilized mainly in an ultrasonic motor of a standing-wave type. Its operational principle is to drive a rotor with the substantially elliptical movement of a transducer. Such motors move the mover by applying frictional force between each mass point of the mover and the transducer, which is held against the mover with preset pressure. The standing-wave type piezoelectric transducer is easily constituted to realize highly efficient vibration. Compared with a traveling-wave type motor, the standing-wave type motor has an advantage of high efficiency and high output.
The piezoelectric transducer utilized in a conventional standing-wave type motor is a mechanical resonance type. In order to excite an elliptical movement with high efficiency, it uses mechanical resonance having a normal vibration mode making a one-way vibration on the contact surface of an elastic member with an exciter, and making an elliptical vibration on the contact surface of the elastic member with the mover. Such a piezoelectric transducer presents two main problems.
First, it is difficult to excite a one-way vibration with same frequency on the elastic member which vibrates in two directions without exciting the other directional vibration due to slight constructural asymmetry between the elastic member and the exciter. Therefore, the transducer of high efficiency and large output has not been obtained. Also in order to excite a single vibration on the elastic member, the constitution of the transducer was largely restricted.
In addition, the piezoelectric transducer utilized in the standing-wave type ultrasonic motor simultaneously excites a two-directional vibration substantially intersecting a resonator fixed on an electromechanical conversion element to obtain a substantially elliptical vibration. Conventionally, the transducer is fixed by supporting at the position of matching nodes of the two-directional vibration.
However, a second problem with the prior art is that the shape of the resonance is largely restricted to match the nodes of the two-directional vibration. The combination of vibration modes is also restricted.